This invention relates to recovery of liquid and gaseous products from subterranean formations containing oil shale, and more particularly, to techniques for forming a void space in an in situ oil shale retort site in preparation for explosively expanding formation within the retort site.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. The term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods that have been proposed for processing oil shale involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirements for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,596; 4,043,597 and 4,043,598, which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale wherein such formation is fragmented by explosive expansion techniques to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort.
In forming such a fragmented mass, at least one void is excavated from formation within the retort site, leaving a remaining portion of unfragmented formation within the retort site adjacent the void. Explosive is loaded into blasting holes drilled in the remaining portion of unfragmented formation. The explosive is detonated for explosively expanding the remaining portion of unfragmented formation toward the free face of formation adjacent the void for forming a fragmented permeable mass of formation particles containing oil shale in an in situ oil shale retort.
During retorting, hot retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the fragmented mass and introducing an oxygen-supplying gaseous combustion zone feed into the fragmented mass to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By continued introduction of the combustion zone feed into the fragmented mass, the combustion zone is advanced through the fragmented mass.
The combustion gas and the portion of the combustion zone feed that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone. This heats the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting", in the oil shale. The kerogen decomposes to gaseous and liquid products, including gaseous and liquid hydrocarbon products, and to a residual solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort. An off gas also is withdrawn from the bottom of the retort. The off gas contains combustion gas, including carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
U.S. Pat. Nos. 4,043,595 and 4,043,596 disclose methods for excavating a void space within a retort site in preparation for explosively expanding formation containing oil shale for forming an in situ oil shale retort. According to a method disclosed in those patents, formation within the retort site is excavated to form a columnar void in the form of a narrow vertical slot bounded by unfragmented formation having a vertically extending free face within the retort site. Blasting holes are drilled adjacent the slot and parallel to the free face, the blasting holes are loaded with explosive, and the explosive is detonated for explosively expanding unfragmented formation within the retort site toward the slot for forming a fragmented permeable mass of formation particles containing oil shale in an in situ oil shale retort. In one embodiment, the vertical slot is centered within the retort site, and the height of the slot extends essentially the entire vertical dimension of the retort being formed, and the length of the slot extends essentially the entire distance between opposite side walls of the retort being formed. The slot can be over 200 feet in height, over 120 feet in length, and about 18 feet wide.
In one embodiment, a vertical slot is excavated by initially drilling and forming a 4-foot diameter raise the height of the retort being formed. The raise is bored in the center of the slot being formed, and rows of blasting holes are drilled on opposite sides of the raise. The blasting holes are the same height as the slot being formed. The blasting holes are loaded with explosive, and such explosive is detonated in increments to explosively expand formation toward the free face provided by unfragmented formation surrounding the raise for enlarging the raise in steps progressing lengthwise along the slot being formed. Drilling and blasting sequences are repeated until the length of the slot is enlarged the full width of the retort being formed. Explosive is then loaded into unfragmented formation remaining within the retort site adjacent the void and detonated in a single round for explosively expanding the remaining formation toward the slot for forming the fragmented mass. A more complete description of the techniques for forming the slot is disclosed in U.S. Pat. Nos. 4,043,595 and 4,043,596.
U.S. Pat. Nos. 4,043,597 and 4,043,598 disclose an alternative method for forming a void space within a retort site in preparation for forming a fragmented mass. According to a method disclosed in those patents, three vertically spaced apart horizontal voids are excavated within the boundaries of a retort site. Separate vertically spaced apart horizontal retort level access drifts are excavated on upper, intermediate and lower levels of the retort site. A separate rectangular horizontal void is excavated at each retort access level so that a respective retort access level drift extends to and from the horizontal void being formed. The horizontal cross section of each horizontal void is substantially similar to that of a retort being formed. Blasting holes are drilled in upper, intermediate and lower zone of unfragmented formation adjacent the horizontal voids. Explosive is loaded into the blasting holes, and the explosive is detonated in a single round for explosively expanding the zones of unfragmented formation toward the horizontal free faces of formation adjacent the voids for forming a fragmented mass. Further details of techniques for forming retorts using such horizontal void volumes are more fully described in U.S. Pat. Nos. 4,043,597 and 4,043,598.
In certain instances, oil shale deposits can lend themselves to development according to principles of this invention wherein a void space can be formed within an in situ oil shale retort site in lieu of excavating a vertical slot or a plurality of horizontal voids as described above. Within an oil shale deposit there can be an extremely large number of generally horizontal deposition layers containing kerogen, as well as other minerals and rock materials interspersed between the layers containing kerogen. Some of these other minerals or rock materials can occur in deposition layers or strata several feet thick. These other minerals or rock materials can be softer than formation containing kerogen, or they can occur as relatively loose or porous deposition layers when compared to kerogen which can occur in more compact or solid deposition layers. For example, mineral deposits such as tuff can occur in oil shale deposits, and tuff is softer than formation containing kerogen which occurs in compact or solid deposition layers. Layers of gravel also can be interspersed throughout formation containing oil shale, and such layers are more loose or porous when compared with compact or solid kerogen deposition layers. Formation strata containing fractured oil shale, including fractured kerogen deposits, also can occur in formation strata throughout an oil shale deposit. It can be desirable to form a void space within zones of formation containing such soft, loose, porous or fractured material in lieu of, or in combination with, forming a void space by excavating a vertical slot or horizontal void volumes as described above. Such relatively weak deposition layers are commonly low in kerogen content and therefore the waste resulting from forming the void spaces is relatively low. It can also be desirable, particularly in a horizontal void volume system, to form voids for placement of explosive in weak deposition layers which commonly extend horizontally through the formation, i.e., parallel to the free face toward which formation is explosively expanded.